70# with total stack of 0.266"....sounds like it is a pretty radical design to get that much weight with that low of a stack. What does the profile look like?
On a straight longbow or a mild r/d, I'd say go ahead and keep narrowing the limbs until you drop weight, but 70 down to 55 is a big drop and if this is a real radical design, I don't know if it could be done. It might help if you could post a pic of the bow's profile and maybe someone else can offer up better advice.